1. Field of the Invention
This invention relates to a pulse generator for use in a speed sensor or the like.
2. Description of Related Art
FIG. 1 is a sectional view of a conventional pulse generator disclosed, for example, in Japanese Utility Model Application Laid Open No. 62-33986 (1987). The numeral 1 designates a casing of the pulse generator to cover a voltage generating coil 2 which is wound around the body of a bobbin 3 having flanges at both ends in the axial direction thereof and four leg portions. A magnetic core 4 penetrates the center of the body of the bobbin 3, with one end thereof protruding outside the casing 1. The protruding end of the core 4 is a detecting head 4a to detect an object to be detected, i.e., a gear (not shown) or the like passing through. The other end of the core 4 has a larger diameter than the other portions of the core 4, thereby stopping the magnetic core 4 to slip off from the bobbin 3. A bias magnet 5 is mounted in contact with the stopper portion of the core 4 and a spacer 6 to increase the magnetic flux. The bias magnet 5 and spacer 6 are placed inside the leg portions of the bobbin 3.
At an outer periphery of the leg portions of the bobbin 3 is provided a terminal 8 which connects the coil 2 with a lead 7 extending from the pulse generator to an external device. A starting end of the lead 7 is crimped by a crimping portion 8b at one end of the terminal 8. Another end portion of the terminal 8 is an entwined portion 8a where the end portion of the coil 2 wound around the bobbin 3 is entwined so as to fasten the coil 2. The entwined portion 8a the last end of the coil 2 fixed to by soldering is flexible. The coil 2 between the bobbin 3 and terminal 8 is rigidly secured and protected by tapes 9 at the position where the winding is finished and at the entwined portion 8a, respectively.
FIG. 2 is an enlarged plane view of the entwined portion 8a from the body of the bobbin 3 in the course of twining the end portion of the coil 2 therearound. FIG. 2 shows the state that the end portion of the coil 2 is twined round the entwined portion 8a which is raised approximately at right angles to the leg portions of the bobbin 3 and fixed to the entwined portion 8a by soldering. Then, the entwined portion 8a is stretched towards the bobbin 3. Accordingly, the coil 2 where it is finished winding is suitably loosened. The crimping portion 8b and the lead 7 are sealed in a mold member 10 which has leg portions 10a reaching inside the leg portions of the bobbin 3. The leg portions 10a press the spacer 6 and bias magnet 5 against the slip-preventing portion of the magnetic core 4 to fix them.
The pulse generator having the above-described structure operates as follows. When the gear (not shown) is rotating, with teeth thereof passing through the vicinity of the detecting head 4a, the magnitude of reluctance in a magnetic circuit formed by the edge of the gear, magnetic core 4 and a gap defined by the distance between these edge and core changes most, whereby the quantity of the magnetic flux crossing the coil 2 changes most and the pulse generator outputs the largest voltage. On the other hand, when a trough portion between the gear teeth passes through the vicinity of the detecting head 4a, the magnitude of reluctance in a magnetic circuit formed by the trough portion, magnetic core 4 and a gap defined by the distance between trough and magnetic core 4 changes least, whereby the quantity of the magnetic flux crossing the coil 2 changes least and the pulse generator outputs the smallest voltage. Accordingly, the output voltage of the pulse generator forms approximately a sine wave.
In the conventional pulse generator described above, when the entwined portion 8a is inclined in assembling, the coil 2 from the bobbin 3 to the terminal 8 is pressed back towards the terminal 8 because it is fixed by the tape 9 at the bobbin side thereof. As a result of this, a stress is applied to the solder joint, the joint being susceptible to breakage because of the solder being wet around the coil 2, resulting in the joint being broken causing cutoff of the thin coil.
Moreover, after the coil 2 is wound up around the bobbin 3, the finishing side of the coil 2 is fixedly bandaged with the tape 9 so as not to loosen. The winding tension of the coil disadvantageously slackens before bandaging the tape 9, thus making it difficult to fix the coil 2 by the tape 9. Further, a vibrating shock or the like during handling of the bobbin 3 in assembling the pulse generator or during use of the pulse generator cause cutoffs of the coil 2 by being rubbed at an edge of the flange of the bobbin 3.